MPG decrease in very cold weather??

[edyvw]

Originally Posted by supton
What's so hard about an ice scraper and cracking a window? That is what I do, unless if there was a storm and I have an inch of ice; in that case yeah I'll let it idle so as to soften the ice. But for frost? scrape and motor off.

I leave the heat off until I see the temp gauge move. Is it hurting anything to turn the heat on full blast right after getting in? Nope. Technically the heater core acts like a mini-radiator (same principle) only the real radiator has a thermostat that won't open when the coolant is below a temperature around boiling--but the heater core gets coolant always, so turning on the heater fan is going to slow down engine heating up. Does it really matter? I'm not sure--the engine will warm up faster with it off, but once on, it's still drawing heat away, so I'm not sure it matters... me, I just don't want cold air blowing on me, so I wait until I know it'll be warm air blowing on me.

My truck drops mpg badly in winter. I drove it year round once and could hit 20 mpg, 21 even, in summer. Snow tires and winter dropped it to 16 mpg. Although now that I drive it little it seems to get 16 year round... my other cars will drop in cold weather but it seems like, if it's above 20F I don't have much of a change. But for me, a short trip is 15 minutes, most trips are much longer--once up to temp, winter losses are not massive.

IIRC winter blend gas often has a bit less Btu's per gallon, so there's something like up to 5% there (but it varies with the blend).

Dramatically! Also depends on the vehicle, but it will slow down arm up process.

 

[DGXR]

My thoughts... Engines warm up quicker when they are driven gently, significantly faster than just idling. Allow at least 30 seconds for the oil to circulate, and however long it takes the fast idle to come down so the transmission doesn't lurch when you engage the gear. Also make sure the windows are clear before you drive away (obvious, but need to say). Because your engine doesn't get so cold in summer, it will reach full temp quicker and you will notice a significant increase in fuel economy... all else being equal.
Don't put a heavy load on a cold engine, or rev it too high. (I keep mine below ~3000 rpm until it's nearly full temp.) Do some research on open loop/closed loop operation. Seasonal fuel blends, if there is such a thing in your region, will have an effect on fuel economy.

 

[emg]

Originally Posted by joekingcorvette
Good info here. I had no idea turning on the heater would slow down engine warmup.


Our Forester will ramp up the engine rpms when the heater is on and the engine is cold (which seems to mean it's under 70-80C). Instead of a typical 1200rpm in city driving, it sits at 2000rpm.

This means I get about the same mpg in winter as I would in summer, so long as I keep the heater off. But that's only an option down to about -20C, then the mpg drops about 30% due to the higher rpms with the heater on.

It's really quite bizarre to see the engine rpms rise when I take my foot off the gas and it shifts from acceleration mode to warming the engine up mode.

 

[SwampSurvivor]

My 2011 Focus was real bad. On a cold start, that engine was going to run at 2500 - 3000 RPM. It didn't matter or care what you were trying to do. I could drive it part of the way to work without touching the gas pedal (in some cases, I'd have to hit the brake on the road because it would try to maintain 3000 RPM in gear). Also made it hard to shift in the cold. I'm sure it didn't help the pilot bearing that failed.

I really like the way my Subaru Forester 6MT handles it. It does the fast idle thing, but once it realizes you're moving, it will idle down normally and doesn't rev back up when I'm trying to shift or at a stop light.

The 17 Leaseon Sentra will idle less than 1500 RPM, but the CVT holds the revs at 2000-2500 when cold driving around.

Higher idle is only part of it. Gear oil , transmission oil and such contribute to it. And longer on a richened fuel mixture. I have 3 differentials in the Subaru running 75w-90 that have a ton of resistance in the cold. I can definitely feel it.

 

[pitzel]

Okay some real world data. Drove Regina to Minneapolis at 50mph. Normally my car gets 38 USmpg in the summertime on those roads and at those speeds. In -30C weather last week, the efficiency was reduced to 34 USmpg. Car hasn't had its oil changed in 100k km's (~62k miles), but using 0W-30 oil (which has probably thickened to a -40 oil).

GM 3.1L V6 (LH0 engine) / 4T60 transmission. Heater running full blast for the first hour or two, and then gradually throttled back as everything inside the car was toasty warm.

 

[road_rascal]

Originally Posted by pitzel
Car hasn't had its oil changed in 100k km's (~62k miles), but using 0W-30 oil (which has probably thickened to a -40 oil).





Say what now?

 

[pitzel]

Originally Posted by road_rascal
Originally Posted by pitzel
Car hasn't had its oil changed in 100k km's (~62k miles), but using 0W-30 oil (which has probably thickened to a -40 oil).


Say what now?


Documented in other threads... Complete with pictures of both the oil pan removed (oil drained and refilled) as well as valve covers removed.

 

[circuitsmith]

Originally Posted by pitzel
Normally my car gets 38 USmpg in the summertime on those roads and at those speeds. In -30C weather last week, the efficiency was reduced to 34 USmpg.


I figure the difference in air density and wind resistance is a big chunk of that.
From -30C to 25C is more than 10% air density change.

 

[Cujet]

Originally Posted by JLTD
All of the above, plus don't forget that before fully warmed you burn more fuel as well.


There are a number of reasons for this.

1) The air/fuel ratio must be richer (more fuel) when the engine is cold, this compensates for lack of fuel vaporization.
2) The cold engine has thicker oil and more internal friction
3) The cold engine transfers more of the heat of combustion into the cold metal parts, instead of expanding the air and pushing the pistons down.

 

[RDY4WAR]

Originally Posted by Cujet
Originally Posted by JLTD
All of the above, plus don't forget that before fully warmed you burn more fuel as well.


There are a number of reasons for this.

1) The air/fuel ratio must be richer (more fuel) when the engine is cold, this compensates for lack of fuel vaporization.
2) The cold engine has thicker oil and more internal friction
3) The cold engine transfers more of the heat of combustion into the cold metal parts, instead of expanding the air and pushing the pistons down.


Yep. I'm part of a big group of 80s-90s Mustang owners and regularly see people replacing the OEM 192°F thermostat with a 160°F stat and no tuning changes. For the OBD-I cars, it doesn't initiate closed loop until ~175°F. You can tune it to changeover sooner, but most don't. They only pickup +2-4 hp and lose -2-5% in fuel economy. The few I got to do a UOA after 4-6k miles all had >2% fuel dilution despite mixed highway / long trips. The car nevers gets out of open loop and keeps dumping fuel trying to heat the engine up.

To make matters worse, several insist on staying away from ethanol blended fuel and only use ethanol-free 93 aki fuel which has a higher boiling point and lower RVP compared to E10 which only makes vaporization worse.